g 80 LIMITS OF DISTRIBUTION. 



The nature of the soil may constitute an insuperable obstacle to a permanent 

 occupation of a particular district by plants, and so act as a check to dispersion. 

 Everywhere localities with sandy, loamy, or rocky subsoils alternate with loose, wet, 

 and porous argillaceous earths. And yet how utterly different are the conditions 

 under which plants growing on these two kinds of soil respectively must exist. Let 

 us consider the case of a particular species, whose seeds are uniformly scattered 

 over a district which includes areas with different kinds of soil. In the parts where 

 the ground possesses the requisite properties for the maintenance of the species in 

 question, the seedlings are able to establish a firm footing, whilst those seeds which 

 fall on uncongenial soil perish. If millions of fertile seeds belonging to a marsh- 

 denizen were scattered over a dry tract of land, not a trace of them would be found 

 at the end of a twelvemonth. The extent to which the chemical in addition to 

 the physical properties of the soil operate, in producing this result, and the part 

 played by competition between different plants for possession of the ground, have 

 been already dealt with (p. 495 et seq.). From these observations it is obvious- 

 also that the distribution of species, even within a district of restricted area, is 

 materially influenced by the soil, and that the spots in such a district where a- 

 particular species thrives and multiplies are divided from one another by tracts- 

 where it does not exist. Those restricted sites in a locality, which offer favourable 

 conditions to the progress of a particular species, and allow of its posterity main- 

 taining possession of the soil, where, indeed, the species is permanently established 

 are called the habitats of that species. The botanists of former times distinguished 

 such habitats into a large number of different classes, from which we may select the 

 following as the most important: fresh- water springs (f antes), salt springs (salina), 

 brooks (amTies), torrents (torrentes), rivers (fluvii), pools (stagna), lakes (lacus), the 

 sea (mare), shores of rivers and lakes (ripce), sea-coasts (littora), marshes (uliginosa\ 

 swamps which dry up in the summer (paludes), peat-bogs (turfosa), places that are 

 periodically flooded (inundata), pastures (campi), steppes (pascua), deserts (deserta), 

 sunny hills (colles), stony places (lapidosa), rocky places (rupestria), sands (arena\ 

 argillaceous soil (argilla), loam (lutvm), debris (ruderata). Sufficient has been said 

 to prove the fact that these habitats undergo various displacements, and are some- 

 times entirely lost, in consequence of changes effected in the soil in course of time 

 through the action of running water and aerial denudation, or in consequence of 

 the accumulation of humus. 



The most potent influence affecting the dissemination and distribution of plants- 



that exercised by climate. The length of the days and corresponding duration of 



the sun's illumination, the temperature of air, ground, and water at the different 



seasons of the year, the condition of the atmosphere in respect of moisture, the 



quantity of water deposited by the atmosphere, and the times at which such deposi- 



ion occurs in each year, the strength and direction of prevailing winds-not on! 7 



; ese circumstances in general of the greatest moment to plant life, but each 



> factor stands in a definite relation to each species. If the fruits or brood- 



l a plant are carried by any of the usual agencies of dispersion to a place 



